There is a growing need for personal protective apparel that guards against toxic chemical and biological agents. These agents may be                (a) accidentally released in a chemical manufacturing plant, in a scientific or medical laboratory or in a hospital;        (b) released intentionally during wartime by a government to attack the military forces of the opposition; or        (c) released during peacetime by criminal or terrorist organizations with the purpose of creating mayhem, fear and widespread destruction.        
For this reason, the United States military and other defense organizations of countries all over the world have sought to provide adequate protection against chemical and biological warfare agents. The need for such protective apparel also extends to police departments, fire departments, emergency responders and health care providers.
According to the Handbook of Chemical and Biological Warfare Agents (D. Hank Ellison, CRC Press, Boca Raton, Fla., 1st edition, 1999), most chemical warfare toxins are fatal at concentrations as low as 1 part per million (ppm). Hence, to provide adequate protection from chemical warfare agents, a protective suit has to be almost impermeable to such chemicals. It is not difficult to devise structures that are impermeable to toxic chemical vapors and liquids, but such structures are also hot, heavy and uncomfortable to wear. The degree of comfort offered by a protective suit is largely determined by the amount of water vapor that can permeate through the protective fabric. The human body continuously perspires water vapor as a method for controlling body temperature. When a protective fabric hinders the loss of water vapor from the body, the transpirational cooling process is hindered, which leads to personal discomfort. When a protective suit allows little or no loss of water vapor, extreme heat stress or heat stroke can result in a short period of time. Hence, in addition to offering the highest levels of protection against toxic chemicals and liquids, a practical chemical and biological protective suit must have high water vapor transmission rates. The appropriate protective structure must also be light in weight and offer the same high level of protection over a long period of time.
There are two major problems with some protective garments described in prior art. First, some known protective garments offer the same constant level of protection at all times. In most situations, the wearer of a protective garment does not require protection from the environment at all times. Protection is only needed when a toxic chemical or biological agent is present in the environment. Second, not all of the garments described in the art offer an optimum balance of protection and comfort. Typically, either comfort is sacrificed at the expense of protection or vice versa.
A need remains for protective structures in which at least some of the aforementioned problems are overcome by using a membrane structure having movable membranes, one advantage of which is a variable and controllable permeability.